Spectrophotometric Determination of Osmium with Pyrogallol G. H. FAYE Mineral Sciences Division, Mines Branch, Department of Mines and Technical Surveys, Ottawa, Canada
b Pyrogallol has proved to be a satisfactory reagent for the spectrophotometric determination of osmium. A stable blue color with an absorption maximum a t 585 rnb is produced when chloro- or bromo-osmate (IV) a t a pH of approximately 3 is heated for a short time in a boiling water bath with excess reagent. Using 1 -cm. cells, the recommended concentration range is from 1 to 15 p.p.m. of osmium in aqueous solution and from 0.4 to 6 p.p.m. when the blue complex is concentrated by extraction into n-amyl alcohol. The osmium-pyrogallol system has been studied over the pH range 1 to 5.5, and the existence of several anionic complexes is indicated.
B
of the structural and functional group similarity of pyrogallo1 (1,2,3-benzenetriol) to pyrocatechol (1,2-benzenediol), a spectrophotometric reagent for determining osmium ( I ) , a n investigation of the possible similar use of pyrogallol was considered to be worthwhile. I n solutions having a p H in the range 2.5 to 3.5 and containing a large excess of pyrogallol, osmium forms a blue complex that has an absorbance maximum near 585 mb. This complex is suitable for analytical purposes, and optimum conditions for its formation have been established. The color system is highly stable and reproducible, and the recommended concentration range is from 1 to 15 p.p.m. of osmium in an aqueous medium. The sensitivity of the method can be increased by extracting the blue complex with amyl alcohol, in which case the optimum concentration range is 0.4 to 6 1i.p.m. of osmium. ECAUSE
EXPERIMENTAL
Apparatus. All absorbance measurements were made with a Beckman 111o d el D TJ spectrophotometer equipped with 1.OO-cm. Cores cells. Reagents. STANDARD OSMIUM S o L u n o s was prepared by dissolving a n accurately weighed quantity of Johnson, M a t t h e y and Co. L t d . ammonium chloro-osmate in 0.5*V hydrochloric acid a n d diluting the solution to a known volume with acid of the same concentration. The ammonium chloro-osmate salt was analyzed
gravimetrically for osmium by direct reduction in hydrogen and this gave 43.6% osmium as compared to the theoretical 43.30/,. PYROGALLOL SOLUTION2% was prepared by dissolving 2 grams of reagentgrade pyrogallol in 100 ml. of distilled water. Several drops of 12N hydrochloric acid were added to the solution to inhibit atmospheric oxidation of pyrogallol. This reagent should be prepared fresh daily. BUFFER SOLCTION(approx. p H 2) was prepared by mixing 100 ml. of 4 N sodium acetate solution with 106 ml. of 4.47 hydrochloric acid. Procedure. (a) COLORDEVELOPMEKT. Place in a 250-ml. beaker a n aliquot of chloro- or bromo-osmate containing up to 350 pg, of osmium. Add 1 ml. of 2.501, sodium chloride solution and evaporate the sample just to dryness. Dissolve the salts in 10 ml. of water and then add 1 ml. of buffer solution and 5 ml. of 2% pyrogallol solution. Using a p H meter, adjust the p H to 2.9 to 3.1 by the dropwise addition of a saturated solution of sodium bicarbonate. Transfer the sample solution to a 25-ml. volumetric flask (without washing) and heat it in a boiling water bath for approximately 30 minutes. (The maximum color develops within 10 to 15 minutes but as a precaution the longer heating time is preferable.) Cool the flask to room temperature (in running water) and dilute the contents to volume with water. After mixing, allow the sample to stand for 15 minutes and then measure its absorbance a t 585 mp against either water or a reagent blank. (The absorbance of the reagent is negligible in the wavelength range 500-900 mp.) (b) EXTR.4CTIOK O F OSMIUM COMPLEX. When less than 100 p g . of osmium is to be determined, transfer the 25 ml. of blue solution from (a) above, to a 60-ml. separatory funnel and extract the osmium complex with two separate 5-ml. portions of n-amyl alcohol. Combine
Table 1.
Osmium, p.p.m. 1 94
5
82 9 70 11 64
13.58
Calibration Data and Sensitivity
Absorbance at 585 mp 0 003
Absorptivity, p.p.m.-' cm.-' 0 048
0 27.1
n
0 453 0 548 0,640
0 047 0 047 0,047
I
-
_
047
the extracts and centrifuge for a short time to remove water droplets. Mea.ure the absorbance against water or a reagent blank a t 585 mp and determine the amount of osmium present by reference to a calibration curve prepared from data obtained from standard osmium solutions that have been taken through the above extraction procedure. RESULTS AND DISCUSSION
Spectral Absorption Curves. T h e absorption spectra for the blue complex, in both aqueous and amyl alcohol solutions, are shown in Figure 1. The peaks of these spectra are broad and from these the wavelength of maximum absorbance was taken to be 585 mb in both the aqueous and the amyl alcohol media. Adherence to Beer's Law and Sensitivity. The results shown in Table I indicate t h a t Beer's law is followed by solutions containing a t least up to 14 1i.p.m. of osmium when the rolor is developed by the recommended procedure. The color of the solutions produced in the tests with 5.82 and 9.70 p.p.m. of osmium were stable for a t least four hours. Extraction. Because the sensitivity of the osmium-pyrogallol system in aqueous solution is relatively low for some applications, the possibility of concentrating the colored species by extraction into organic solvents was investigated. The blue osmium complex is extractable in n-amyl alcohol, butanol, methyl isobutyl ketone, and tributyl phosphate (T.B.P.). Methyl isobutyl ketone vias unsatisfact,ory because it contained oxidizing agents t'hat reacted with pyrogallol to produce an intense brown color. XIthough T.13.P. was the most efficient solvent' test'ed, it was rejected because the absorbance of the extract' increased gradually with time. Amyl alcohol was chosen as the most suitable solvent because a clear extract with a stable color can be produced. Tests showed that 92% recovery is achieved consistently when 25-nil. volume of aqueous solution containing u p to 480 pg. of osmium i R extracted with VOL. 37, N O . 2, FEBRUARY 1965
259
I
0.60
I
I
I
I
I
1
t
0.3
0
1.0
2.0
3.0
4.0
5.0
6.0
OH
Figure 2. Effect of pH on absorbance a t 585 mp (vs. water) 5.8 p.p.m. of Os; heating time, 30 min. 300
400
500
600
700
BOO
900
mu
Figure 1. plexes
Absorption spectra of osmium-pyrogallol com-
Blue complex (585 mp), 11.6 p.p.m. of Os, pH 3 Blue complex ( 5 8 5 mp), 9.7 p.p.m. of Os in amyl alcohol extract Blue complex ( 6 6 5 mp), 11.6 p.p.m. of Os, p H 1 . Brown complex ( 4 7 5 mp), 11.6 pap.m. of Os, pH 5.5
1.
2. 3. 4.
two separate 5-ml. portions of n-amyl alcohol. A linear calibration curve is obtained for the combined extracts (10 ml.) from samples containing at least up to 150 pg. of osmium (the useful upper limit of concentration) thus indicating that the extent of extraction is constant when the above procedure is used. The spectral absorption curve for a typical amyl alcohol extract is shown in Figure 1 (Curve 2). The blue osmium-pyrogallol complex is not extractable in chloroform or carbon tetrachloride, and this is in contrast with the chloroform-soluble osmiumcatechol complex formed by Jasim et al.
0).
Study of Variables. EFFECTOF TEMPERATURE A N D RATE OF COLOR DEVELOPMENT.At room temperature and a t a p H of a b o u t 3, there is no apparent color development in solutions containing u p to 14 p.p.m. of osmium and a large excess of reagent during the first 15 minutes after sample preparation; however,
Table II.
when such solutions are heated in a boiling water bath, the maximum color (at 585 mp) is developed within 10 to 15 minutes. As a precautionary measure, a heating period of approximately 30 minutes is suggested in the recommended procedure. EFFECT OF pH. To determine the optimum p H range for color development, a series of solutions each containing 145 pg. of osmium was taken through the procedure recommended above, with each solution being adjusted to a different pH value with either sodium bicarbonate solution or dilute hydrochloric acid after the addition of 1 ml. of buffer solution. Figure 2 shows the absorbance values (at 585 mp) plotted against p H and from this it is evident that the pH should be in the range 2.5 to 3.3 for maximum color development. EFFECT OF AMOUNT OF REAGENT. Experiments showed that, with solutions containing up to 350 pg. of osmium per 25-ml. final volume (14 p.p.m.), a volume of a t least 4 ml. of 2% pyrogallol
Effect of Foreign Ions
Element (as
chlorocomplex) Platinum( IV) Palladium(I1 1 Rhodium( 111) Iridium( I T ) Ruthenium( I T ) Iron( 111) Copper( 11)
Nickel( 11)
260
Concn., p.p.m.
Absorbance
20 23
0.007
0.133
22
0.068
20
12
20
20 20
ANALYTICAL CHEMISTRY
0.015 0.160 0.015
0.020 0.000
Color produced Yellow Greenish yellow (cloudy) Amber Pale yellow Bluish purple Pale yellow Yellow Colorless
solution is required for maximum color development. This corresponds to a mole ratio of reagent to osmium of 50 to 1. Only a slight increase in absorbance is obtained at higher mole ratios. I t wm observed that in tests in which the mole ratio was less than about 15 to 1, a blue-gray color was produced and that the blue color could be separated from the gray by extraction with namyl alcohol. It is probable that the gray material was a colloidal suspension of hydrated osmium dioxide. EFFECTOF DIVERSEIONS. To determine the effect of some foreign ions on color development, a series of tests was performed in which chloro-complexes of certain platinum and base metals were taken through the recommended procedure. The absorbance values at 585 mp and the colors produced in these tests are given in Table
11. Of the ions tested, only those of ruthenium and palladium offer serious interference in the proposed method. EFFECT OF CHLORIDEIONCONCENTRATION. I n certain experiments in which a relatively large amount of hydrochloric acid was present in the sample solution, and which was subsequently neutralized with sodium bicarbonate during adjustment of the pH, somewhat low and erratic results were obtained. Therefore the effect of chloride ion concentration on color development was investigated. The chloride ion concentration was varied in a series of test solutions (each containing 243 pg. of osmium) by adding weighed amounts of sodium chloride just prior to adjustment of the p H to a value of approximately 3 in the recommended procedure for color development. From the absorption spectra of the test solutions, it was apparent that as the chloride ion concentration is increased the maxima are suppressed and shifted to longer wavelengths, thus suggesting the formation of a lower com-
.
plex in which chlorine atoms have replaced one or more pyrogallol molecules. The result's of these experiments indicat'e that for maximum color development the chloride ion concentration should be kept at' a constant low level and this can be accomplished by evaporating the sample solut,ion in the presence of a measured amount of sodium (25 mg.) to drive off free hydrochloric acid and by accurately dispensing the chloride-containing buffer solution (1 ml.). Some Properties of OsmiumPyrogallol Complexes. EXISTENCE OF L O W E RCOMPLEXES.Studies of the osmium-pyrogallol system have shown t h a t at' a p H of approximately 1 a blue complex is formed that' exhibits maximum absorbance near 665 m p and has a low solubility in amy1 alcohol; a brown complex, also wit,h a low solubility in amyl alcohol, and with an absorbance maximum at' about 475 mp, is formed by osmium in the p H range 4.5 to a t least' 6.
Absorption spectra of solutions containing the above-mentioned complexes are shown in Figure 1 (curves 3 and 4). Because of the lower maxima of these curves it is assumed that the pyrogallol to reagent ratio of the corresponding complexes is lower than that of the complex formed at a p H of approximately 3 in the recommended procedure for color development. EXTRACTABILITY OF OSMIUM-PYROGALLOL COMPLEXESIN T.B.P. As judged visually, one 10-ml. portion of T.B.P. will extract nearly completely the osmium-pyrogallol complexes formed in the p H range 1 to 5.5, ANIONICNATUREOF OSMIUM-PYROGALLOL COMPLEXES.T h a t the osmium-pyrogallol complexes are anionic is indicated by their strong retention by the anion exchange resin Amberlite IRA-400. COMBINING RATIOS. Results of experiments using the method of continuous variations ( 2 ) and the mole ratio method ( S ) , suggest the possibility that
the analytically useful complex (585 mp) formed a t a pH of approximately 3 is a 3 to 1 complex of osmium to pyrogallol, and that the complex formed at approximately p H 1 is a 2 to 1 complex. EXAMINATION OF PHLOROGLUCINOL AND RESORCINOL AS REAGENTS.Both phloroglucinol (1,3,5-benzenetriol) and resorcinol (1,3-benzenediol) were ineffective as reagents for osmium under conditions similar to those of the recommended procedure for color development; this is probably because the functional groups are not located on adjacent carbon atoms in these compound. LITERATURE CITED
(1) Jasim, F., Magee, R. J., Wilson, C. L., Mikrochim. Acta 1-2. 11 11962). ( 2 ) Job, P., Ann. Chi&. (I&~me)'(lO)9, 113 (1928). (3) Yoe, J. H., Jones, A. L., IND. ENG. CHEM.,ANAL. ED. 16, 111 (1944). RECEIVEDfor review July 27, 1964. Accepted November 23, 1964.
Correlation and Prediction of Solvent on Paper Chromatographic R, Values KENNETH A. CONNORS School o f Pharmacy, University of Wisconsin, Madison, Wis. An equation is given that relates the R, values for a compound in two paper chromatographic solvent systems. This equation appears to describe with reasonable accuracy many experimental R, values for several classes of compounds and types of solvent systems. By combining the theoretical equation with an experimentally determined reference quantity, fairly accurate predictions can b e made of R, values for a series of compounds in one solvent if the corresponding R, values in a second solvent are known.
T
HE goal of the chromatographer is thp capability of predicting accurately the extent of zone migration in a defined chromatographic system. Many factors operate to control the migration of the zone; the most important of these are the structure of the sample compound, the nature of the solvent system, and the nature of the support. Much attention has been directed to the relationship between molecular structure of solutes and their chromatographic behavior-see, for example, the reviews of Cassidy ( 2 ) and Giddings and Keller ( 4 ) , and the exten-
sive investigations of Green and Marcinkiewicz (b)-and some quantitative correlations have been discovered. These results are of greatest value in structure determination and identification problems. The practicing analyst usually is presented with a mixture of closely related substances to be separated. The most effective way to influence the chromatographic bphavior of such a mixture (since the molecular structures usually cannot readily be altered) is by suitable selection of the solvent system. Many qualitative guide rules for the selection of chromatographic solvents are used; an example is the useful observation that a polar developer will cause faster zone migration of a polar solute than will a nonpolar developer. Recently Soczewinski and Wachtmeister (8,9) have presented some quantitative relationships between paper chromatographic R values and fractional composition for certain types of solvent mixtures. The effect of the support is, of course, critical in adsorption chromatographic procedures. I n this paper only partition chromatographic systems are considered, so the solid support is (ideally) not of great importance, since it serves
mainly to support the statioiiary phase. Actually the support probably also acts in a minor way as an adsorbent for the solute, thus complicating the problem. I n the case of paper chromatography the support is believed to influence the nature of the internal phase markedly. I n seeking correlations between zone migration and solvent systems fewer complications may be expected with column partition chromatographic results than with paper chromatographic data. However, few R values are recorded for column partition studies, while a great many Rf values are available from paper chromatographic separations. I n this paper a semi-empirical correlation of paper chromatographic Rfvalues in different solvent systems is presented. THEORY
The R f value of a compound is related to its partition coefficient, K , and the relative phase volume fraction, U , by Equation 1,
Rj
=
KC KC+1 ~
which expresses the idea that only that fraction of solute that is in the mobile VOL. 37, NO. 2, FEBRUARY 1965
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